FIG. 1 is a perspective view of an engine room 101 of a gasoline engine vehicle (hereinafter, simply referred to as a “vehicle”) 100. An engine 110, an intake manifold 112, an air cleaner 113, a radiator 114, a battery 102, and the like are accommodated in the engine room 101. A 4-cylinder engine is illustrated in FIG. 1.
A plug hole (not shown) is installed in every cylinder of the engine 110, and an ignition plug (not shown) is inserted in each plug hole. A mixture of air which has passed through the air cleaner 113 and the intake manifold 112 and fuel from a fuel tank (not shown) is supplied to each cylinder of the engine 110. The engine 110 is started and rotated by igniting (sparking) an ignition plug at an appropriate timing.
FIG. 2 is a block diagram of part of an electric system of a vehicle 100r. The electric system of the vehicle 100r has a battery 102, an ignition coil 104, an ignition plug 106, an engine control unit (ECU) 108, and an igniter 200r. The ECU 108 periodically generates an ignition signal (ignition timing (IGT)) for indicating an ignition timing of the ignition plug 106 in synchronization with a rotation of the engine 110. A secondary coil L2 of the ignition coil 104 is connected to the ignition plug 106. The igniter 200r generates a high voltage (secondary voltage VS) of tens of kV in the secondary coil L2 by controlling a current of a primary coil L1 of the ignition coil 104 depending on the ignition signal IGT, and discharges the ignition plug 106 to explode a mixer within the engine 110.
The igniter 200 has a switch element 202 and a switch control device 300r. The switch element 202 is, for example, an insulated gate bipolar transistor (IGBT), in which a collector thereof is connected to the primary coil L1 and an emitter thereof is grounded. The switch control device 300r controls a voltage of a control terminal (gate) of the switch element 202 depending on the ignition signal IGT to control ON/OFF of the switch element 202. Specifically, the switch control device 300r turns on the switch element 202 during a period in which the ignition signal IGT becomes a high level. When the switch element 202 is turned on, a battery voltage VBAT is applied across the primary coil L1, so that a current flowing in the primary coil L1 is increased over time. When the ignition signal IGT transitions to a low level, the switch control device 300r immediately turns off the switch element 202 to cut off a current IL1 of the primary coil L1. At this time, a primary voltage VIL1 (=L·dIL1/dt) of hundreds of V proportional to temporal derivatives of the current IL1 is generated in the primary coil L1. At this time, a secondary voltage VS of tens of kV obtained by multiplying a winding ratio to the primary voltage VIL1 is generated in the secondary coil L2.
The switch control device 300r largely has a determination stage 300A and a driving stage 300B. The determination stage 300A receives an ignition signal IGT from the ECU 108 and determines a level (high or low) of the ignition signal IGT. For example, the determination stage 300A includes a determination comparator 302 for comparing a voltage VIN of an input line 301 with a reference voltage VREF to generate a determination signal SDET having a high/low value.
The driving stage 300B switches ON/OFF the switch element 202 depending on the determination signal SDET. A delay circuit 304 provides predetermined delay to the determination signal SDET. The delay amount is set such that a time difference (delay) between a time at which the ignition signal IGT transitions and a time at which the ignition plug is discharged has a predetermined value. A pre-driver 306 and a gate driver 308 control a gate voltage of the switch element 202 depending on an output from the delay circuit 304.
When the ECU 108 normally operates, after the ignition signal IGT becomes a high level, the ignition signal IGT transitions to a low level to ignite the ignition plug 106 after the lapse of an appropriate period of time. However, when the ECU 108 has an error, the ignition signal IGT maintains the high level, rather than transitioning to a low level, so that the switch element 202 is maintained in the ON state. Then, problems may arise such as an increase in heat generated by the switch element 202 or a large amount of current flowing in the primary coil L1 of the ignition coil 104.
In order to solve the above problems, a conduction protection circuit 310 is installed. When the ignition signal IGT transitions to a high level and a predetermined conduction protection time TP has lapsed, the conduction protection circuit 310 forcibly turns off the switch element 202 to ignite the ignition plug 106. FIG. 3A is a waveform view illustrating an operation of the conduction protection circuit 310. When the ignition signal IGT transitions to a high level, the switch element 202 is turned on to increase a coil current (collector current of IGBT). The conduction protection circuit 310 includes a timer, which measures a time duration in which the ignition signal IGT (determination signal STET) becomes a high level. Further, when a count value of the timer reaches a setting value (##) corresponding to the conduction protection time TP, the switch element 202 is forcibly turned on to cut off the coil current IC. In this case, due to the forcible cutoff of the coil current IC, the voltage (secondary voltage VS) of the secondary coil L2 of the ignition coil 104 is significantly changed and the ignition plug 106 is ignited.
In some cases, the ignition of the ignition plug 106 by the forcible OFF of the switch element 202 is not desirable depending on the type of engines and ECUs. In this case, as illustrated in FIG. 3B, a soft shutoff function of gradually turning off the switch element 202 after the lapse of the conduction protection time TP, and gradually decreasing the coil current IC is required.
In order to prevent ignition of turn-off of the switch element 202 according to the conduction protection, it is required to reduce the coil current IC to a long time scale TSSO ranging from tens of ms to hundreds of ms, and to this end, it is required to lower a gate voltage of the switch element 202 from a high level voltage (for example, 5 V) to a low level voltage (0 V) by a time scale ranging from tens of ms to hundreds of ms. This is called a soft shutdown function.